1. Field of the Invention
This invention relates to a protection circuit for a semiconductor device and, in particular, to a circuit for protecting from an overcurrent, a power semiconductor device for use as a switching device, etc., especially a large-current type.
2. Prior Art
In addition to a conventional contact-equipped switch heretofore used, a contactless type semiconductor switch has recently been employed as a device for effecting on-off control of an electric circuit through which a large current flows. This type of switch has the advantages of being free from arcing, noises, and the need for maintenance, but is easily destroyed in the face of an overcurrent, that is, an abnormal current such as a short-circuiting current. For example, if a current exceeding a prescribed level flows through a thyristor constituting a semiconductor switch, then the thyristor is destroyed, producing a fixed "on" state.
In order to prevent the destruction of a semiconductor switch, a quick-break fuse is inserted into the electric circuit whereby, upon the flow of an overcurrent, the circuit is interrupted for the protection of the semiconductor switch. An attempt has also be made, in this field of art, to protect a semiconductor switch by using a contact breaker in place of a quick-break fuse.
However, when the quick-break fuse is broken due to an overcurrent, then it should be replaced by a new one, in order to restore the circuit to its normal state, and this poses a burdensome maintenance problem.
Furthermore, in order for the breaker's contact to be opened for protection of the device, a mechanical operation is involved, causing a time delay to occur from the generation of an overcurrent to the interruption of current flow in the circuit. Therefore, an adequately fast response cannot be obtained, which is required for the protection of a semiconductor device.